CHICAGO (AP) — The Chicago Transit Authority has told employees to take a series of precautionary steps to keep out-of-service, unmanned trains from moving along the tracks.

The guidance was issued late Thursday, three days after an empty, four-car train traveled the wrong way along a Blue Line track before slamming into a parked train at the Harlem stop and injuring dozens of people.

“We wanted to look at adding some redundancies to the controls we already have in place, so we added a few things related to the procedures of storing cars in for repair,” CTA spokesman Brian Steele told the Chicago Sun-Times. “Some of it is re-emphasizing procedures already in place, and some are new things.”

He compared the new safety redundancies to wearing a “belt and suspenders, and another belt.”

The train was in a service yard awaiting repairs when it left the parked area, went past a turnaround and through another station before the accident at the station in Forest Park, about 10 miles west of Chicago.

Video footage shows that no one was driving the four-car train. More than 30 people were treated for minor injuries.

The Blue Line connects Chicago’s near-western suburbs with the city’s downtown and O’Hare International Airport.

Steele told WMAQ-TV the bulletin was initiated by the transit agency, not federal officials who are investigating Monday’s collision